Study over time causes depression (4)
I believe the answer is:
dent
'depression' is the definition.
(synonyms)
'study over time' is the wordplay.
'study' becomes 'den' (term for a study or office).
'over' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'den'+'t'='DENT'
'causes' acts as a link.
(Other definitions for dent that I've seen before include "Mark of knock" , "'Dinge, depression (4)'" , "Surface hollow" , "TV lexicographer?" , "Hollow from impact" .)